1. Field of the Invention
The present invention relates generally to display devices, and more particularly, to improvements in the contrast and reliability of a passive matrix display device.
2. Description of the Background Art
Among display devices having a matrix arrangement, liquid crystal display devices, for example, may be largely divided into passive matrix type devices and active matrix type devices depending upon the driving method. The passive matrix type devices without any active element is simple in structure and less restricted in the manufacturing process, and therefore can be manufactured inexpensively. If, however, nematic liquid crystal which reacts to effective voltage is used in a passive matrix type device, cross talk could be caused between adjacent pixels, and since the pixels are displayed in a dynamic display mode, the contrast is generally low.
Meanwhile, in the active matrix type devices, each pixel is provided with an active element such as transistor, which performs switching operations, and therefore cross talk is restrained. Also in the active matrix type devices, since the pixels are displayed in a static display mode, high contrast generally results. However, an active element should be formed for each pixel, which results in a low yield, and many restrictions exist in the manufacturing process.
As disclosed by Japanese Patent Laying-Open Nos. 64-4721 and 64-17025, the use of a ferroelectric element having spontaneous polarization in a passive matrix type display device of a simple structure has been proposed to provide a display device of a relatively simple structure which could implement high contrast image display.
FIGS. 6B and 6A schematically show one pixel included in a passive matrix type liquid crystal display device disclosed in Japanese Patent Laying-Open No. 64-4721. FIG. 6A is a plan view, and FIG. 6B is a cross sectional view taken along line 6B--6B in FIG. 6A.
In this display device, a first signal electrode 23 of Cr is formed on an upper surface of a lower glass substrate 21. First signal electrode 23 and lower glass substrate 21 are covered with a ferroelectric layer 24 having spontaneous polarization and comprised of amorphous TiBaO.sub.3. To provide a pixel region, a pixel electrode 25 of a transparent ITO (indium-tin oxide) film is formed on ferroelectric layer 24, partially lying over first signal electrode 23. Meanwhile, a second signal electrode 26 of an ITO film is formed on a lower surface of an upper glass substrate 22. A liquid crystal layer 27 is held between lower and upper substrates 21 and 22.
FIG. 7 is an equivalent circuit diagram of a display device having a plurality of pixels arranged in a matrix, each of which is as shown in FIGS. 6A and 6B. In the equivalent circuit diagram, a liquid crystal capacitor 30 and a ferroelectric capacitor 31 are connected in series between a bus line 28 for scanning electrodes in the X-direction and a bus line 29 for data electrodes in the Y-direction. More specifically, first signal electrode 23 corresponds to data electrode 29, and second signal electrode 26 corresponds to scanning electrode 28. Liquid crystal layer 27 and ferroelectric layer 24 correspond to capacitors 30 and 31, respectively.
The ferroelectric capacitor generally exhibits the electrical characteristic where voltage applied (field intensity) and capacitance (load) have a non-linear relation and may therefore serve as a so-called non-linear element. More specifically, if a signal voltage equal to or higher than a threshold value is applied between the bus lines in the X- and Y-directions at the selection of a pixel, the ferroelectric film spontaneously polarizes, such that electric charges may be generated to charge the liquid crystal capacitor. In a pixel selected in one frame, the ferroelectric film has a memory function holding the spontaneous polarization, which permits the charges given to the liquid crystal capacitor to be held until the pixel is reselected in the next frame. As a result, higher contrast image display with less cross talk is enabled as compared to the conventional passive matrix type liquid crystal display device.
However, a glass substrate is generally used as the substrate of such a display, because the substrate is desirably transparent and inexpensive, and has a sufficient area. The use of the glass substrate is however encountered with the following disadvantage.
When we refer to the manufacture of a ferroelectric element usually used as a non-volatile memory on a silicon substrate, for example, a ferroelectric film of an inorganic material such as PZT (lead zirconate titanate) is formed into an amorphous state by means of a sputtering or sol-gel method, then is annealed at a temperature of about 600.degree. C. or higher for crystallization, and the crystallization provides enough ferroelectric characteristic and reliability to the device. In connection with this, the difference in the ferroelectric characteristic between crystallized PZT and amorphous PZT is described, for example, in SPIE, Vol. 1758, Sol-Gel Optics (1992), pp. 261-273. More specifically, the document clearly indicates that the residual dielectric polarization in the amorphous PZT annealed at a temperature of 400.degree. C. is 3.2 .mu.C/cm.sup.2, while that in the crystallized PZT annealed at a temperature of 700.degree. C. is 31.5 .mu.C/cm.sup.2, which is improvement by one order of magnitude in the characteristic.
However, the glass substrate is used in the display as described above, and glass has a softening temperature far lower than silicon. Therefore, it is difficult to crystallize an inorganic ferroelectric film in an amorphous state by annealing at a temperature of about 600.degree. C. or higher on the glass substrate. As a result, an inorganic ferroelectric film having sufficient ferroelectric characteristic and reliability cannot be obtained on such a glass substrate, and the display including the inorganic ferroelectric capacitor as described above has not been yet reduced to practice.
Meanwhile, Japanese Patent Laying-Open No. 64-17025 discloses the use of a film comprised of a copolymer P (VDF/TrFE) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE), both ferroelectric polymers, in place of a ferroelectric film of an inorganic material. Since the P (VDF/TrFE) film may be spin-coated and thereafter crystallized by annealing at a temperature of about 150.degree. C., an organic ferroelectric film may be easily formed on a glass substrate. The actual use of such a P (VDF/TrFE) film formed on a glass substrate for fabricating a display device is reported in SID 91 DIGEST, pp. 18-20 (1991).
Now, in an organic ferroelectric film, dipoles coupled to a principal chain forming a polymer are oriented to cause spontaneous polarization, while the principal chain itself changes its state or rotates at this time. The organic ferroelectric film is changeable in the crystal state and is disadvantageously unreliable, and therefore such a display device using the organic ferroelectric film has not yet been reduced to practice. Note that the inorganic ferroelectric film generates spontaneous polarization as the loaded atoms are displaced even for a small distance, and therefore the film has relatively higher reliability than the organic ferroelectric film.